At present, gaseous nitric oxide (NO) is most commonly measured by mixing a gas sample with ozone gas at low pressures. When a nitric oxide molecule reacts with an ozone (O.sub.3) molecule, it forms nitrogen dioxide (NO.sub.2) and oxygen (O.sub.2) and emits a photon in the process. This photon possesses a red or near-infrared wavelength. The concentration of nitric oxide in the gas sample is thus determined by measuring the intensity of those photons. However, red and near-infrared wavelengths are not detected efficiently by standard photodetecting devices such as photomultiplier tubes and photodiodes. Consequently, special photodetector devices that are more sensitive to red light must be used. These devices must be cooled to subambient levels to reduce background noise from thermal effects. These special devices and their cooling requirements add cost and complexity over that required to measure visible light.
In addition, an ozone-based nitric oxide gas detector requires a vacuum pump and a method for making ozone, which is typically a high-voltage electrical discharge. As a result, ozone-based detectors are generally bulky and complex, and require a significant amount of electricity to operate. The high voltage required to operate an ozone-based detector can pose a safety risk to the user and to those nearby. OSHA regulations restrict allowable ozone exposure, making it difficult to use ozone-based nitric oxide gas detectors in the workplace. Furthermore, ozone is a toxic gas, and it must be vented or destroyed after use. Because ozone is a pollutant, environmental regulations may prohibit venting the ozone in many areas, forcing the user of an ozone-based detector to destroy the ozone after use. Destruction of the ozone adds an additional step, and additional cost, to the nitric oxide measurement process.
Gaseous nitric oxide may also be detected by placing a gas sample in contact with an alkaline luminol solution containing hydrogen peroxide. As with the ozone-based method of detection, the chemical reaction between nitric oxide and the luminol solution results in the emission of photons. Unlike the ozone-based method of detection, these photons possess wavelengths in the more energetic end of the visible light spectrum. While the luminol-based method of detection overcomes some of the problems of ozone-based detection, it possesses drawbacks of its own.
When measuring atmospheric nitric oxide, carbon dioxide levels are typically too low (300-400 parts per million (PPM), which is 0.03-0.04 percent) to interfere with the measurement. However, carbon dioxide typically constitutes several percent of exhaled human or animal breath. This amount of carbon dioxide is orders of magnitude greater than the amount present in the atmosphere. At present, this amount of carbon dioxide interferes with the detection and measurement of nitric oxide in human or animal breath. This interference primarily occurs in three ways. First, at a concentration of several percent, carbon dioxide reacts with the luminol solution to produce the same number of photons produced by the reaction of several parts per billion (PPB) of nitric oxide with luminol, tricking the detector into registering the presence of several PPB of nitric oxide which is not present in the sample. Second, carbon dioxide is known to react with a key intermediate in the nitric oxide/luminol reaction, ionic peroxynitrite (ONOO.sup.-). This reaction reduces the response of the luminol solution to nitric oxide, causing the detector to measure less nitric oxide than is actually present. Third, some gaseous carbon dioxide will dissolve in the alkaline luminol solution, changing its pH and thereby reducing the standing background signal of the luminol solution.